Pulley assemblies comprising a hub adapted to be rigidly connected to a crankshaft of an internal combustion engine and a pulley adapted to cooperate with a belt of the accessory drive and rotationally connected to the hub by means of a spiral spring are known.
Such known assemblies also generally comprise an integrated torsional damper, which consists of an coaxial annular seismic mass at the hub and an elastic connection element arranged between the hub and the seismic mass.
In use, the spiral spring couples the hub and the pulley with the rotation and presents a relatively low stiffness so as to allow to filter the torsional vibrations of the driving shaft and avoid such vibrations from being transmitted to the accessory drive.
In the known pulley assemblies, the pulley is connected radially to the hub and comprises a sliding wall defining a central cavity for accommodating a portion of the hub and the spiral spring. The spiral spring comprises a first end portion connected rigidly to the hub and a second end portion fitted with radial interference against the sliding wall.
During operation, the pulley may be in advance of the hub and the second end portion may slip relatively to the sliding wall when a maximum transmissible torque value is exceeded. Such condition occurs during cranking of the engine when there are considerable fluctuations in torque, or during the instants immediately following a sudden deceleration when the crankshaft turns at a relatively slow angular speed while the accessory drive proceeds by inertia at a higher angular speed.
In operating conditions in which the pulley tends to in advance of the hub, the second end portion slides on the sliding wall. In particular, the spiral spring is generally made of harder material with respect to that of the sliding wall and the consequent wear may reduce the useful life of the pulley assembly.